This study is designed to elucidate the rates and mechanisms of free radical reactions with molecules of biological significance. One part of the study is an investigation of the relative importance of reactions A and B in the attack of radicals on biologically important thiols. This study will be carried out using steady state photolysis methods to generate R equals CH3 and the mechanism will R plus R'SH yield R'S plus RH (A); R plus R'SH yield RSH plus R' (B) be deduced from product analysis. A second study is concerned with the fate of the thiyl radicals produced in Reaction A. Rates of the reactions of R'S will be determined in a flash photolysis apparatus by direct observation of the photolytically produced R'S radicals, or by using a competitive pilot reaction to monitor these reactions. Both alkyl and aryl thiyl radicals will be investigated.